Rheostat



May 24, 1938. H. G. RICHTER I 2,118,267

RHEOSTAT Filed Feb. 25, 1935 wWl INVENTOR Henry G. Riemer ,mmv BY TTORNEYS Patented May 24, 1938 BHEOSTAT Henry G. Richter, Westfield, N. J., assignor, by

meine assignments, to P. R. Mallory & Co. Inc.,

Indianapolis, Ind.,v a corporation of Delaware Application February 23, 1935, Serial No. '1,800

10 Claims.

This invention relates to variable resistors, and more particularly to power rheostats intended to operate at relatively high temperature.

The primary object of my invention is to generally improve variable resistors, especially low resistance units commonly called power rheostats. Such rheostats dissipate substantial power and operate at relatively high temperatures. The insulation core on which the resistance wire is wound must therefore be made of a refractory material. In the past, a core made of porcelain or the like has been used, and inasmuch as the core is commonly made circular or arcuate in configuration, it has been necessary to employ a bobbin machine to wind the resistance wire on thecircular core. However, such a machine is costly and slow in operation.

A more particular object of my invention is'to overcome the foregoing difficulty and to provide a power rheostat the resistance element of which may bev wound straight and thereafter bent to arcuate coniiguration even though using a refractory insulation core. 'Ihis object I 'fulfil preferably by using a Rockbestos core comprising a strip of copper sheathed in asbestos. Such a core is flexible and heat-resistant. Because of the high thermal conductivity of the copper strip, excellent heat dissipation is obtained even when only a small part of the resistance element is in use. The finished element is permanent and durable because the resistance winding itself operates to hold the asbestos covering on the copper strip. For these reasons I prefer to use Rockbestos as the bendable refractory insulation core, but other materials may be used, i'or example laminated mica the laminations of which are secured together by both low temperature and high temperature cements.

Further objects of my invention center about the mechanical construction of the rheostat and` are to provide a stop arm the stopping force of which is applied directly to .the control shaft `without affecting the contact arm; to locate the stop arm within the contact arm for cooperation with a stop molded directly on the base of the unit; to-'provide a collector ring or cup advantageously located inside the stop arm yet connected directly to the contact arm at the outside of the unit; to provide two spaced contact arms with a wiper shoe protectedly located therebetween; to make the wiper shoe wear-resistant and self-lubricating; and to utilize the soldering lugs of the unit for holding the resistance element in place and to act as short-circuiting terminals To the accomplishment of the foregoing and other objects which will hereinafter appear, my invention consists in the rheostat elements and their relation one to the other as are hereinafter more particularly described in the specification and sought to be defined yin the claims. The specification is accompanied by drawing in which:

Fig. 1 is a rear elevation of a rheostat embodying features of my invention;

Fig. 2 is a section taken in elevation ir. the plane of the line 2-2 of Fig. l;

Fig. 3 is a section taken in the plane of the line 3 3 of Fig. l;

Fig. 4 is a section through the wiper shoe take in the plane of the line 4-4 of Fig. 3; I

Fig. 5 shows a step product or part of the rheostat and is explanatory of the invention; and

Fig. 6 is a modification.

Referring to the drawing and more particularly to Fig. 5, I employ a core strip I2 made of refractory insulation material. This strip is preferably straight and is wound while in straight condition with a resistance wire winding, as is indicated at I4. The strip may then be severed into appropriate lengths which are bent to the circular or arcuate shape indicated at I6 in Fig.

uration 'and to thereafter apply the resistance wire winding'thereto. In accordance with my invention it is possible and desirable to rapidly wind long strips of insulation core, say, 6 feet in length. j

The core l2 preferably comprises a metallic strip I8 sheathed with asbestos 2li. Strip I8 may be made of copper which is pliable and an excellent conductor of heat. The asbestos may be temporarily held in place by cotton thread or the like, and even if such thread is later burned away, no harm results because the asbestos is then securely held in place by the resistance wire winding I4. If desired, the core I2 may be purchased in iinished form under the commercial name Rockbestos.

Referring now to Figs. 1 through 4 of the draai` ing, the rheo'stat comprises a refractory insulation base 22 molded, for example, from porcelain. This base has an annular flange 2li which acts as a supporting wall for the resistance element it. The resistance element is placed around flange 2&1 and is held in place by soldering lugs the inner ends of which are reversely bent to U shape and slipped over the resistance element flange. More specifically, the lugs are bent to provide an. outer wall 28, a 'nach wall and an inner wall 32, as is best shown in Figs. l and 2,. Lugs are secured to base by rivets 3G passing therethrough. The end of the resistance wire winding is electrically connected to the soldering lug, preferably by the use of hard or silver solder.

`The back wall may be slit to form a T and the end of the resistance wire winding may be passed beneath wall and anchored about the neck of T as is best shown in li, preparatory for the soldering operation. lt should be understood that the two lugs 2G are alike and similarly connected to the adjacent ends oi the resistance element, although only one of the lugs is exposed in Fig. l by supposed removal or" the enamel coating applied to the finished unit.

Guter wall oi' lug 2b is preferably further used as a short-circuiting terminal for the resistance element. For this purpose, the edge @il oi wall 28 is struck inwardly or beveled, thereby facilitating movement of the wiper or contact shoe t2 from the resistance wire to the terminal.

The-base 22 with applied resistance element I6 is next dipped in a protective enamel which not only coats the unit but lls in the interstices between the resistance wire windings and between the resistance element and iiange 24. The enamel is wiped away on the outside of the resistance element, thereby exposing the windings for contact with wiper shoe 42. The unit is then air-dried and finally baked or fused at a suitable high temperature, say, 1200 Fahrenheit.

Coming now to the control mechanism of the rheostat, base 22 is provided with a central tubular support 44. A threaded metallic bushing 46 is passed through support 44 and locked in place between flanged head 48 and a nut 5o. A lock washer 52 may be inserted between head 48 and support 44 in order to prevent rotation of bushing 46.

Bushing 4S receives a control shaft 54 the inner end 56 of which may be squared and is securely riveted directly to an appropriate stop arm 58. This stop arm cooperates with a stationary stop 59 preferably molded integrally with base 22 within flange 24. Control shaft 54 is axially located by means of a thrust washer 60 beneath stop arm 58, and a split washer 62 received in a mating groove 84 on control shaft 54.

Control shaft 54 has lnsulatedly mounted thereon contact arms 10 and 'l2 which cooperate with wiper shoe 42. located outside stop arm 52 and at the outside or back of the unit. Electrical connection is elotained to contact arm 10 through collector brushes i4 located inside stop arm 58 and within the unit. To this end I secure a disc of insulation 'i6 on the outside of stop arm 58 by means of rivets 18, and then mount a metal cup 88 over` insulationdisc 16. The side wall 82 or cup 8@ acts as a collector ring against which brushes "i4 bear. As is shown in Fig. 3,` side wall 82 is cut awayV at 84 to receive stop arm 58 but without contacting the stop arm. Cup 8b is secured to insulation disc 16 by means of rivets 8S, and these rivets may also be used to lock contact arms lb, 'i2

The contact arms are preferably i in physical electrical engagement do. in fact, because arms lli, are preferably struck from a single piece o1 relatively heavy gauge sheet metal, it may be said that cup is riveted in place between insulation disc lli and the junction or common portion Sil of arms "lil, l2, as is best shown in Fig. 2. Both the cup 89 and junction Sil are out away to clear the rivets l@ which hold insulation disc ld to stop arm 58, as indicated in l. i

Arms il@ and 'i2 may, ii desired, be connected near their outer extremity by a` web 9o, the projecting ends 92 and all are bent forwardly or at angles outside the resistance element. While i have referred to the arms lo and lll as though two spaced arms are used, it shouldbe understood that the construction may equally well be described as including a single broad aim the outer extremity oi which is cut away between the parte and Qt.

The wiper shoe d2 is located in the space between the ends Q2 and lili. This shoe is prefn erably made of a block oi copper graphite alloy because the said alloy is self-lubricating and is capable oi. substantially indefinite wear. The bottom face 8b oi the blocl (see fl) is preferably made straight so that it contacts with only one or two of the resistance wire windings Ml at any one time. rlhe block is self-adjustably seated against the windings by means oi a leaf spring 98 having a rounded projection B00 bearing against a mating seat M2 on the outside of the block. Leaf spring 88 is secured to the end 92 of contact arm it by means of a rivet H04. To insure good electrical connection, a thin l'exible strap `|06 is connected at one end to shoe 42 and at its other end to contact arm l0, as by the use of aforesaid rivet H34.

In the particular form here shown, the end 94 of arm 12 is not connected to shoe 42 and acts only as a protective guard for said shoe during operation of the rheostat. It should be understood, however, that if desired the leaf spring 98 :may be connected to both ends 92 and 94, or the leaf spring may be connected to end 92 and electrical conductor IBB connected to the end 94, and so on.

Brushes 'i4 are the ends of a resilient U-shaped strip of metal H0 the center portion of which is riveted at M2 to the rearwardly bent inner end H4 of a third or center soldering lug H6 which passes from the inside to the outside of the unit between the ends of the resistance element and through an opening or passageway in annular flange 24 of the base. Center lug IIS is riveted in place by a rivet H8. It will be understood that the three soldering lugs shown make it possible to vary the resistance in either direction or to use the unit as a potentiometer.

While the bendable refractory core l2 of the resistance element is preferably made as before described, it is possible to use other materials, and a core made of laminated mica lis illustrated in Fig. 6. The core $28 consists of a large nurnber of thin laminations of mica which are preferably secured together by two kinds of cement one eiective at low temperatures, and the other effective at high temperatures. l am informed and believe that the low temperature cement is an organic cement, which is volatilized and driven off at the nal high baking temperature, while the high temperature cement is an inorganic cement, which iirct fluxes and becomes effective at the high baking temperature. A. suitable product is made by the New England Mica Company under lil ing with thecost of a casing and increasing the heat radiation from the unit; the supporting flange or wall holds the otherwise bendable resistance element rigidly in truly circular configuration; while the impregnation and coating with enamel not only protects the structure but gives the same the appearance of being made in one piece.

It will be apparent that while I have shown and described my invention in preferred forms, many changes and modifications may be made in the structures disclosed, without departing from the spirit of the invention, dened in the following claims.

I claim:

1. A variable resistor comprising a control shaft, a stop arm mounted on the. end of said shaft, an insulation disc secured to said stop arm, a metallic cup mounted over said insulation disc, the side wall of said cup being cut away to receive said stop arm, a contact arm conductively secured to said cup, a stationary brush slidably contacting said cup, and a stationary stop cooperating with said stop arm.

2. A variable resistory adapted for high temperature. operation, comprising a refractory insulation base, an arcuate approximately cylindrical wire-wound resistance element mounted on said'base, at least the. exterior of the core of said element being made of refractory insulation, the

windings extending transversely of said core,

fused enamel securing said windings to said core and securing the wound core to the base, the

Iwindings being exposed at the outer cylindrical periphery of the 'resistance element, a contact arm movable around said resistance element, a contact shoe made of a block of copper graphite alloy and having a contact face of substantial dimension in two directions, and resilient means secured to said arm and self-adjustably supporting 'the contact shoe in engagement with the exposed windings at the outer cylindrical periphery of the resistance element.

3. A variable resistor comprising an arcuate approximately cylindrical wire-wound resistance element the windings of which are exposed at the outer cylindrical periphery thereof, a control shaft for the resistor, two spaced metallic contact arms extending divergently from the shaft and movable around the outside periphery of said resistance element, a contact shoe made of a block of copper graphite alloy having a relatively large flat bottom face bearing against the exposed windings, a leaf spring secured at one end to one of said arms and extending toward the other arm and having a rounded protuberance bearing against the aforesaid contact shoe, and a flexible connector strip extending from said contact shoe to said arm, said shoe being located between the two spaced arms.

4. A variable resistor adapted for high temperature operation and comprising a refractory insulation base having an annular flange projecting therefrom, an arcuate wire-wound resistance element mounted on said base around the outside of said flange, the core of said element being a thin bendable strip at least the exterior of which is made of refractory insulation, soldering lugs riveted to said base and having their inner ends bent to U shape around the resistance element and ange in order to hold the resistance element on the base and against the ange, means connecting the ends of the resistance. wire to said soldering lugs, and fused vitreous enamel filling all of the space between and bonding together the resistance element and base and flange into a unitary heat-dissipating structure.

5. A variable resistor comprising an insulation base having an annular insulation flange projecting therefrom, an arcuate wire-wound re. sistance element mounted on said base against said flange, the core of said element being a strip at least the exterior of which is refractory insulation, soldering lugs riveted to said base and bent around the resistance element and flange at points immediately adjacent the ends of the windings in order to hold the resistance element on the base, means connecting the ends of the resistance wire tov said soldering lugs, the edges of said lugs bearing against said windings being inwardly bent to afford free sliding movement of a contact shoe from the windings to the lugs, whereby said lugs are adapted to act as metallic short-circuiting terminals for said resistor, and fused enamel bonding the resistance element'and base into a unitary heat-dissipating structure.

6. A low resistance variable resistor adapted to operate at high temperature, said resistor comprising a. refractory insulation base having a wide annular support wall, a thin wide bendable core strip at least the exterior of which is made of refractory material, said strip being bent to arcuateshape and mounted on said base outshape around the resistance element and support wall in order to hold the resistance element on the base against the support wall, fused enamel additionally securing said wound core strip on said baseagainst said support wall, all of the space between the wound core strip and base and support wall being solidly filled with vitreous enamel in order to obtain maximum thermal conductivity between the resistance wire and the base, whereby the base acts as an effective radiating surface for heat dissipation from the wire, and movable contact mechanism arranged for movement over the wide outside cylindrical surface of said resistance winding.

7. A low resistance variable resistor adapted for high temperature operation and' comprising a refractory insulation base having an annular support wall, an arcuate approximately cylindrical wire-wound resistance element secured to said base against the outside of said support wall,

. a contact shoe having a relatively large dat bottom face bearing against the exposed windings on the outer periphery of the resistance element, said contact shoe being located between and being protected by the aforesaid two spaced arms, and resilient connection means extending between said contact slice and at least one of salti arins, said resistor coing open and. exposed at outer periphery. l

il. .a low resistance variable resistor atlante@ to operate at high temperature, said resistor comprising a porcelain or refractory insulation base having a wide annular support wall,v wide bendalolecore strip bent to arcuate approximately cylindrical shape and mounted onsalci base out side of and against said support wail, a resistance wire winding on said core, and movable contaet mechanism arranged for movement over the witle outside cylindrical surface of said resistance winding, saicl core ioeing a relatively wide thin strip of highly neat-conductive metal surrounded icy a sheath ci refractory insulation suoli as asbestos, the resistor being coated with fused enamel which covers the base and resistance element except on the outside cylindrical surface where it is ena caged lof the movable contact, and which enamel nils the interstices between tice resistance wire windings and between the resistance element and the base and support wall, thus forming a unitary heat-clissipating structure.

9. A low resistance variable resistor adapted to operate at high temperature, saicl resistor coinnrising a porcelain or refractory insulation base having an annular support wall, a loendable core strip bent to arcuate approximately cylindrical shape and mounted on saicl base outside of anni against said support wall, a resistance wire winding on said core, and movable contact mechanism for movement about said resistance winding, said core being relatively wiile thin strip oi' ioirl'ily neatmconcluctive metal sinrountlerl by sheath oi refractory insulation such as aslnestor, tl'ie resistor being coated with fused vitrcous enamel which covers the resistance element eacept where engaged by the movable contact,

vmethod. which includes preiiminarily sheathing a heat-conductive metallic strip of substantial widtln and length with asbestos, windingl said sheathed strip with a resistance winding, loendu saicl strip to arcuate approximately cy1indrical shape, mounting said arcuately bent strip on a 'oase matie oi refractory insulation, coatingr the structure with a protective enamel which coats the unit and iills in the interstices between the resistance wire windings and between the resistance element and the Tease, wipingj away the enamel on one side of the resistance element to expose the windingsior contact with a wiper shoe, air-drying the enameled unit, and leaking the same at, a suitable extremely nigh temperature, to form the windings, strip, and base into a sturdy integral structure ci goed neat-dissipating characteristic.

HENRY G. RICHTER. 

